Nymphal tick with a pencil for scale. (Credit: Kelly Oggenfuss / Cary Institute of Ecosystem Studies)
More Than A Third Of Blacklegged Ticks Carry Disease, And One Parasite Is Spreading
In A Nutshell
- A nine-year New York study found more than one in three blacklegged ticks infected with at least one disease-causing pathogen, with Lyme disease present at every site, every year.
- Babesia microti, the parasite behind a serious malaria-like illness called babesiosis, was found in more than one in five ticks, far above rates reported in earlier regional studies.
- About 13% of ticks were carrying two or more pathogens at once, and Lyme-infected ticks were more likely to also carry Babesia than chance alone would predict.
- Chipmunks played a larger role in shaping tick infection rates than previously appreciated, alongside white-footed mice.
Walk through the woods in the northeastern United States and there is a reasonable chance any tick that latches on is carrying something dangerous. A nine-year study of blacklegged ticks in New York forests found that more than one in three were infected with at least one disease-causing pathogen, and in peak years, more than half were. Lyme disease was everywhere, as expected. But a lesser-known blood parasite stood out as the study’s most surprising result.
Researchers tested more than 2,000 nymphal blacklegged ticks collected over nine years across six forested sites in Dutchess County, New York. Overall, 38.8% harbored at least one pathogen. Lyme disease bacteria turned up at all six sites every single year, with the odds of a tick carrying the Lyme bacterium increasing by 5.7% per year over the study period. More notable was the rate of Babesia microti, a microscopic parasite responsible for a malaria-like illness called babesiosis. It showed up in more than one in five ticks sampled, far above prior New York estimates and well above nearby regional reports. The findings were published in the journal Ecosphere.
Babesiosis can cause serious illness in people, and for years it has been overshadowed by Lyme disease. This data suggests that oversight may need to change.
Blacklegged Tick Infection Rates Far Exceeded Prior Studies
Previous surveillance in New York typically found Babesia microti in roughly 4% of blacklegged ticks, with Connecticut estimates around 7.5%. In this study the average was 21.4%, and in 2015 it spiked to 42% of all sampled ticks. The findings fit with broader reports that babesiosis has become more common across the Northeast in recent years.
Part of what may be driving those numbers involves Babesia’s complicated relationship with the Lyme bacterium. When a tick is already carrying Lyme, it appears to become more hospitable to Babesia microti, making dual infection more likely than chance alone would predict. Ticks carrying both pathogens were detected every single year of the study, and that pairing was more common than statistically expected in seven of the nine years. About 13% of all ticks in the sample, more than one in eight, were carrying two or more pathogens simultaneously.
Researchers also detected Powassan virus, though only in a small number of ticks: in 2021, infected nymphs were found on two separate grids. Powassan is rare but serious, and its presence in the same forested system adds another layer worth monitoring. Importantly, the Powassan-positive nymphs in this study were not found as coinfections with Lyme or Babesia. Two Rickettsia species, bacteria that cause spotted fever illnesses and are not commonly associated with blacklegged ticks, also turned up. Seven of the 16 pathogens screened for were detected at least once over the nine-year period.
How the Tick-Borne Disease Study Was Conducted
Each year, researchers dragged cloth across the forest floor at six mixed-oak plots to collect host-seeking ticks, and separately trapped and individually marked white-footed mice and eastern chipmunks, recording how many larval ticks were feeding on each animal. All 2,017 nymphal ticks were tested at the Upstate Tick Testing Laboratory at SUNY Upstate Medical University using a sensitive RNA-based method capable of detecting 16 pathogens at once. Small mammal trapping at the site dates back to 1992, giving the research team an unusually deep well of ecological data to draw from.
Nymphal ticks were the focus because they are the life stage most likely to bite people. Roughly the size of a poppy seed, nymphs are easy to overlook and are responsible for the majority of human tick-borne disease cases.
Mice and Chipmunks Both Shape Tick-Borne Disease Risk
Blacklegged ticks pick up infections during their larval stage, before they are large enough to bite humans, by feeding on small forest mammals. Researchers tracked how the abundance of mice and chipmunks on each plot, and how many larval ticks were feeding on each individual animal, predicted infected tick numbers the following season.
When mouse populations were high, individual mice carried fewer ticks because feeding demand was spread across more hosts. Despite this, high mouse abundance still predicted more infected nymphs overall the following year. More mice meant more larvae fed and ready to mature into the ticks that eventually bite people.
Chipmunks played a distinct and underappreciated role. How many larval ticks were feeding on chipmunks mattered more than chipmunk head counts alone, particularly for Lyme infection prevalence and for Babesia-related infected tick density. Counting mice helps estimate how many ticks will emerge in a given season, but the infection rate within those ticks depends on a broader web of forest animal interactions, with chipmunks carrying more weight in that equation than their modest profile might suggest.
Nine years of data from these Dutchess County forests tells a consistent story: blacklegged ticks in these forests are carrying pathogens at higher rates than earlier research from the region indicated, and babesiosis has moved well past the status of a rare footnote.
Disclaimer: The findings in this article are based on a single peer-reviewed study conducted at one forested property in Dutchess County, New York. Results may not be representative of tick populations in other regions. This article is intended for general informational purposes only and does not constitute medical advice. Consult a qualified healthcare provider regarding tick-borne disease prevention, symptoms, and treatment.
Paper Notes
Study Limitations
Conducted entirely within forested plots on a single property in Dutchess County, New York, the study has geographic constraints that limit how broadly its findings apply to other regions or habitat types. Predictive models consistently underestimated infection rates in years when tick pathogen loads were at their highest, pointing to ecological factors not fully captured by the variables measured. Climate was not included in the analysis, despite its known influence on tick survival and host population dynamics. Primers used to detect Anaplasma phagocytophilum cannot distinguish between human-active and non-human strains, so the precise human health risk from that pathogen could not be determined. Roles of other mammalian and avian species beyond mice and chipmunks were not fully evaluated.
Funding and Disclosures
This research was supported by grants from the Long Term Research in Environmental Biology program of the U.S. National Science Foundation (grant numbers DEB-1947756, DEB-1456527, and DEB-0949702), an EPA STAR grant (83489701), the John Drulle, MD Memorial Lyme Fund, Inc., the Upstate Foundation (fund ID: 23709), and a SUNY Empire Innovation Professorship. Funders had no role in study design, data collection, analysis, or preparation of the manuscript. The authors declare no conflicts of interest.
Publication Details
The study was authored by Shannon L. LaDeau, Kelly Oggenfuss, Alexander Schmidt, Saravanan Thangamani, and Richard S. Ostfeld, representing the Cary Institute of Ecosystem Studies in Millbrook, New York, and the Upstate Tick Testing Laboratory at SUNY Center for Vector-Borne Diseases, Upstate Medical University, in Syracuse, New York. It was published in the journal Ecosphere, Volume 16, Issue 12, 2025, under article number e70508. The DOI is https://doi.org/10.1002/ecs2.70508. The paper is open access under a Creative Commons Attribution License.
